Structure and function of the elastic organ in the tibia of a tenebrionid beetle

Abstract

Many insects have a pair of claws on the tip of each foot (tarsus and pretarsus). The movement of the pretarsal claws is mediated by a long apodeme that originates from the claw retractor muscles in the femur. It is generally accepted that the pulling of the apodeme by the muscles flexes the claws to engage with a rough surface of a substrate, and the flexed claws return to their initial position by passive elastic forces within the tarso-pretarsal joint. We found that each tibia of the tenebrionid beetle Zophobas atratus had a chordal elastic organ that tied the apodeme to the distal end of the tibia and assisted the pulled apodeme to return smoothly. The elastic body of the elastic organ consists of a bundle of more than 1000 thin fibrils (0.3-1.5 μm in diameter) with a hairy yarn-shaped structure made by assemblies of intricately interwoven microfibers. Both ends of the fibrillar elastic body were supported by clusters of columnar cells. Ablation of the elastic organ often disturbed the rapid and smooth return of claws from a flexed position when the tarsal segments were forced to curve in order to increase the friction between the apodeme and surrounding tissues in the segments. The result suggests that rapid claw disengagement is an important step in each cycle of leg movements, and the elastic organ may have evolved to assist the reliable detachment of claws that engage tightly with the substrate when climbing or traversing inverted surfaces.

Effects of force detecting sense organs on muscle synergies are correlated with their response properties

“…Our study has presented evidence that the TrF joint membrane is a elastic composite that can serve as a muscle antagonist (Neff et al, 2000): 1) the membrane is reversibly stretched during movements imposed in a posterior direction, 2) the joint shows rapid elastic recoil to the default position after displacement; 3) the joint membrane shows blue fluorescence and toluidine blue staining, consistent with the presence of resilin. Elastic structures that act as muscle antagonists have been identified in the legs of cockroaches (Frazier et al, 1999; Neff et al, 2000; Picker et al, 2012), stick insects (Bässler, 1983), beetles (Ichikawa et al, 2016; Nadein and Betz, 2016) and other insects (Burrows and Sutton, 2012; Michels and Gorb, 2012). These structures minimize inertia (Weis-Fogh, 1960) and can also potentially serve to dampen the effects of postural perturbations (Noah et al, 2004; Dudek and Full, 2009).…”

Force dynamics and synergist muscle activation in stick insects: the effects of using joint torques as mechanical stimuli

“…Engagement with the substrate also requires activation of synergist muscles at different leg joints. The pretarsal claws of insects are anatomically anisotropic and can only exert concentric, pulling forces (Gorb 2008;Ichikawa et al 2016). Concentric forces also enhance adhesion of the pretarsal arolium (Labonte and Federle 2013).…”